1. Field of the Invention
The present invention relates to a high resistance dual antiparallel (AP) pinned spin valve sensor and, more particularly, to such a spin valve sensor wherein various layers of the spin valve sensor have a high resistance for increasing the output signal of the sensor.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is parallel to the ABS, is the position of the magnetic moment of the free layer structure with the sense current conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layer structures are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layer structures. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer structure rotates from a position parallel with respect to the magnetic moment of the pinned layer structure to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
In addition to the spin valve sensor the read head includes nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic layers that are separated by a coupling layer with magnetic moments of the ferromagnetic layers being antiparallel. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.
The dual spin valve sensor has a ferromagnetic free layer structure which is located between nonmagnetic first and second spacer layers. The first and second spacer layers are, in turn, located between first and second pinned layer structures and the first and second pinned layer structures are located between antiferromagnetic first and second pinning layers. The pinning layers pin magnetic moments of the first and second pinned layer structures. It has been found that the magnetoresistive coefficient dr/R of the dual spin valve sensor is about 50% greater than the magnetoresistive coefficient dr/R of a single spin valve sensor. Unfortunately, the sheet resistance of the dual spin valve sensor to the sense current IS is less than the sheet resistance of the single spin valve sensor which seriously reduces the output signal of the dual spin valve sensor. The output signal is essentially a product of the sheet resistance times the magnetoresistive coefficient dr/R.
The free layer structure in a dual spin valve sensor is typically a nickel iron layer which is located between first and second cobalt iron layers. It has been found that a cobalt iron free layer next to a spacer layer increases the magnetoresistive coefficient dr/R and that the nickel iron layer reduces the uniaxial anisotropy HK of the free layer structure so that it is more responsive to signal fields from the rotating magnetic disk. These free layers, however, reduce the sheet resistance of the dual sensor. Further, the nickel iron free layer is employed for reducing a positive magnetostriction of the free layer structure which is caused by the first and second cobalt iron free layers. After lapping the magnetic head to form the ABS, the head is in compression at the ABS which, in combination with the positive magnetostriction, urges the magnetic moment of the free layer structure perpendicular to the ABS. This causes the free layer structure to be unstable when it is repetitively subjected to signal fields from the rotating magnetic disk. Accordingly, the nickel iron is necessary to reduce positive magnetostriction so that the free layer structure will have improved stability. Further, it is desirable to make each of the pinned layer structures in the dual spin valve sensor an antiparallel (AP) pinned layer structure which has an antiparallel coupling layer located between ferromagnetic first and second antiparallel (AP) pinned layers. An AP pinned layer structure is desired because of its thermal stability when subjected to heat in the presence of magnetic fields which urge the magnetic moments of the AP pinned layers from their pinned orientation. Because of partial flux closure between the first and second AP pinned layers of each AP pinned layer structure the exchange coupling field between the pinning layers and the AP pinned layer structures is significantly greater than the exchange coupling field between pinning layers and single pinned layer structures. Unfortunately, the three layers in each AP pinned layer structure shunt more of the sense current IS than single pinned layer structures which causes the dual spin valve sensor to have a lower sheet resistance.
The present invention provides a dual spin valve sensor which has a higher sheet resistance. The free layer structure in the present invention is a single free layer which is composed of cobalt iron nickel (CoFeNi). While the cobalt iron (CoFe) has a positive magnetostriction, the nickel (Ni) reduces the positive magnetostriction to zero, if desired. Further, the resistance of the free layer is significantly greater because nickel iron (NiFe) does not have to be added in order to reduce the positive magnetostriction. Because of the thinner cobalt iron nickel (CoFeNi) free layer the overall sheet resistance of the dual spin valve sensor is increased. Still further, I have discovered that the nickel maintains the free layer in a polycrystalline state. This polycrystalline state promotes polycrystalline structures for each of the layers fabricated on the cobalt iron nickel (CoFeNi) free layer. In a preferred embodiment, the cobalt iron nickel (CoFeNi) free layer is Co70Fe15Ni15.
Another aspect of the invention is to increase the resistance of each of the first and second AP pinned layer structures. This is accomplished by making each of the first and second AP pinned layers of each of the first and second AP pinned layer structures of cobalt iron (Co50Fe50). This composition has 50% more moment than the typical AP pinned layers which are composed of cobalt iron (Co90Fe10). Since each cobalt iron (Co50Fe50) AP pinned layer has 50% more moment than a cobalt iron (Co90 Fe10) AP pinned layer the cobalt iron (Co50Fe50) AP pinned layer can be 25% thinner than the cobalt iron (Co90Fe10) AP pinned layer. Because of this reduced thickness the resistance of each AP pinned layer in each of the first and second AP pinned layer structures is increased so as to increase the overall sheet resistance of the dual spin valve sensor.
An object of the present invention is to provide a high resistance dual antiparallel (AP) pinned spin valve sensor.
Another object is to provide a high resistance free layer and/or high resistance first and second antiparallel (AP) pinned layer structures for a dual pinned spin valve sensor.
A further object is to provide a method of making of the aforementioned spin valve sensors.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.